1. Field of the Invention
The present invention relates to a television tuner. More specifically, the present invention relates to a television tuner employing an acoustic surface wave device in either or both of a high frequency amplifier and a local oscillator.
2. Description of the Prior Art
FIGS. 1 and 2 each show a schematic diagram of a typical conventional television tuner. Each tuner comprises at least a high frequency amplifier RFA, a local oscillator OSC and a mixer MIX. The high frequency amplifier and the local oscillator are structured such that a high frequency and a local oscillation frequency corresponding to any desired television channel may be tuned by tuning circuits in the high frequency amplifier and the local oscillator, whereby any desired channel is received. Referring to FIGS. 1 and 2, the high frequency amplifier RFA is shown comprising a high frequency amplifying transistor TR1, the local oscillator OSC is shown comprising a local oscillating transistor Tr2, and the mixer is shown comprising a mixing transistor Tr3. A tuner thus structured comprises tuning circuits T1 and T2 in the high frequency amplifier RFA and a tuning circuit T3 in the local oscillator OSC, structured to be selectively tuned to the high frequency and the local oscillation frequency corresponding to the desired channel.
FIG. 1 shows a schematic diagram of a tuner of a well known mechanical switching scheme, wherein each of the above described three tuning circuits T1, T2 and T3 comprises a multiplicity of coil segments, totaling three times the number of channels. In such a mechanical switching scheme, the coil segments are coupled to a mechanical switching apparatus, such as a rotary switch, turret switch, and the like, such that desired coil segments are selectively switched to attain tuning frequencies corresponding to a desired television channel. Therefore, such a tuner employing a mechanical switching scheme requires an increased number of coil segments to cover all the television channels. Another problem is caused that the tuner suffers from poor contact in a mechanical switching scheme because of air pollution, mechanical wear and the like, resulting in a reduced life time and a noise. Since a multiplicity of coil segments are required, a tuner becomes large sized and the structure becomes complicated. Furthermore, frequency alignment is required for each channel, which degrades productivity of such a tuner from the practical point of view.
On the other hand, in order to eliminate the above described shortcomings in such a tuner employing a mechanical switching scheme, there has been proposed an electronic tuner basically comprising tuning circuits T1, T2 and T3 each employing a voltage controlled variable capacitance diode, as shown in FIG. 2. FIG. 3 shows an equivalent diagram of each tuning circuit T1, T2 or T3 shown in FIG. 2.
Referring to FIGS. 2 and 3, the tuning frequency of each tuning circuit T1, T2 or T3 is adapted to be variable through the variation of capacitance across a voltage controlled variable capacitance diode Vd. The capacitance of this diode is variable as a function of a reverse bias voltage applied thereacross by way of a tuning voltage. Accordingly, such an electronic tuner requires a tuning voltage which is changed in a stepwise manner so as to correspond to each television channel for the purpose of selection of a desired television channel. Generation of a tuning voltage changeable in a number of steps, however, requires a complicated circuit. In addition, maintaining the tuning frequency at a prescribed value in order to attain a screen image of high quality and thus maintaining a tuning voltage at an exact value further makes the circuit complicated. However, a voltage fluctuation cannot be avoided despite a circuit of high quality, this voltage fluctuation gives rise to a frequency fluctuation. A voltage controlled variable capacitance diode is subject to a time dependent change of capacitance characteristic, requiring the readjustment of the tuning voltage generator. Such an electronic tuner further necessitates a complicated circuit for providing a band switching voltage for selecting a UHF or VHF band, or selecting a high or low band in the VHF band. As well known, a television tuner has to meet the requirement of a high selectivity characteristic wherein a signal having a band width of several MHz is received with fidelity while other extraneous signals are removed. However, the the quality factor Q of a voltage controlled variable capacitance diode VD makes if difficult to increase the quality factor Q of an electronic tuner.
The present invention is aimed to eliminate the above described shortcomings in a conventional tuner by employing an acoustic surface wave filter. In this context, a tuner of interest is seen in U.S. Pat. No. 3,858,118 issued Dec. 31, 1974 to Michael R. Daniel. The referenced patent merely discloses and claims a television tuner employing acoustic surface wave filters in place of conventional tuning circuits implemented by a coil and a capacitor. Thus, the referenced patent does not contemplate any improved channel selecting scheme suited for selection of acoustic surface wave filters, inasmuch as the referenced patent merely shows a diagram of a mechanical switching scheme for selection of acoustic surface wave filters. Thus, there is still room for improvement in channel selection in such a television tuner employing acoustic surface wave filters.